1. Field of the Invention
The present invention resides in a process for preparing aromatic alkylating catalysts and a process for using the same.
The synthesis of alkylated aromatic hydrocarbons by reacting a methylating agent with toluene is not a new concept. Processes are available for producing various mixture of hydrocarbons, such as, xylene isomers, polyalkylbenzenes, etc. The relative amount or extent to which one or more of the above-described products is obtained is determined and/or controlled by the type catalyst, proportions of reactants, and reactor conditions. Catalysts which have been used in the past to prepare xylenes, benzenes, etc. are those selected from acidic cogels, acid-promoted kieselguhr, and various crystalline zeolite aluminosilicates.
Zeolites are described as a three-dimensional network of structural units consisting of silicon-centered SiO.sub.4 and aluminum centered AlO.sub.4 of a tetrahedral configuration, the tetrahedra being interconnected by a mutual sharing of oxygen atoms, the structural arrangement of which forms cavities or cells forming crystalline channels or pore openings having a uniform diameter characteristic of each variety or type of crystalline zeolite.
Silica polymorphs, such as silicalite, which are a major component in our aromatic alkylating catalysts, have a novel topological type of crystalline structure composed of silicate tetrahedra connected in a framework to form a three-dimensional system of intersecting channels defined by 10-ring members sufficiently wide enough to absorb molecules up to 6 .ANG. diameter. Silicalite is hydrophobic and organophilic, and selectively adsorbs organic molecules over water.
The conversion of an aromatic, for example, toluene, to a xylene, such as para-xylene is normally a tedious and time-consuming procedure, many times involving a series of steps. Additionally, catalysts which possess acceptable activity generally tend to give a wide spectrum of products, for example, alkylated aromatics and hydrocarbons having a broad distribution of carbon atoms. This not only complicates the separation and recovery of the desired product, but results in reduced yield of said desired product and erosion of reactants in the production of undesired by-products. The catalysts and process herein are particularly suited to the selective formation of para-xylene from toluene and a methylating agent.
2. Description of the Prior Art
The reaction of toluene with a methylating agent in the presence of a catalyst to produce alkylated aromatics is appreciated and disclosed by the prior art. However, most known processes produce an undesirably large mixture of alkylated aromatics in addition to the desired product.
For example, U.S. Pat. No. 4,002,698 teaches a process for methylating toluene to selectively produce paraxylene by contacting toluene with a methylating agent under methylation conditions in the presence of a crystalline alumino-silicate zeolite catalyst.
Another process for alkylating aromatic hydrocarbons is set forth in U.S. Pat. No. 4,025,572; particularly, the reference discloses a process for the conversion of alcohols or ethers to olefinic and aromatic hydrocarbons in the presence of a crystalline aluminosilicate zeolite. This process is described as suitable for simultaneously producing olefinic hydrocarbons and mononuclear aromatics with high selectivity for para-xylene formation.
U.S. Pat. No. 4,061,724 relates to a crystalline silica composition which is described as selective in adsorbing organic materials from water in either the liquid phase or vapor phase. The crystalline silica is described as suitable for removing organic compounds from waste water.